Masks used in the semiconductor industry comprise a pattern deposited on a surface of a substrate. In performing lithography the mask is positioned at an object focal plane of a projection system and illuminated, so that the pattern is transferred to a photoresist born on a lithography work piece (e.g., a semiconductor wafer). In the case of deep ultraviolet lithography (as opposed to extreme ultraviolet lithography), a transmissive mask which selectively transmits light according to a pattern of opaque material born on its surface is used. The substrate generally used for transmissive semiconductor lithography masks is a deep UV grade of fused silica. The pattern born on the fused silica mask generally comprises Chromium. Such, chrome on Glass (COG) mask blanks are available commercially from Hoya, Inc. of Japan. A direct write e-beam exposure tool is used to expose a resist coated on the Chromium according to a predetermined pattern. The resist is then developed, and the predetermined pattern is transferred to the Chromium by etching.
Unfortunately, mask production is not a very high yield process. Because e-beam exposure is very time consuming, it is preferable to repair defective masks, rather than making new replacement masks. Mask defects can be placed in two categories: (1) undesired absence of pattern material, and (2) undesired presence of pattern material. In order to eliminated unwanted pattern material, laser ablation can be used. For example, U.S. Pat. No. 6,090,507 assigned in common with the present invention teaches a method of laser ablation using a high intensity femtosecond laser ablation tool. Using a femtosecond laser for ablation offers the advantages of complete metal removal and low quartz damage. Femtosecond laser ablation requires a special type of optical source. In particular, mode-locked Titanium doped Sapphire (Ti:Sapphire) based lasers are used to produce high intensity femtosecond pulses.
To fix the opposite problem-unintentional absence of metal, one conventional method involves first using a vacuum deposition apparatus to deposit copper on the mask, then using a focused ion beam tool to decompose a gold compound in order to deposit gold in a predetermined pattern, and finally etch removal of the copper. The copper which must be deposited and then removed, adding to the complexity of the process, is necessary to avoid charging of the substrate being repaired, which would deflect the ion beam from the intended pattern. The process requires two separate vacuum processing steps, i.e., for depositing the copper and for depositing the gold, making it very time consuming.
Laser Induced Chemical Vapor Deposition (LICVD) has been used to deposit metal on substrates. In one conventional LICVD method, a substrate is enclosed in the presence of an organometallic compound which includes a metal to be deposited, (e.g., trimethyl-aluminum for depositing Aluminum). A focused laser is then trained on a point of a substrate, at which it is desired to deposit metal. The laser heats a localized area of the substrate surrounding the point, and causes organometallic molecules which contact the area to thermally decompose and deposit metal. If the laser operates at a wavelength at which the mask substrate is transmissive, then heating must be initiated by focusing the laser on pattern material that is already present (unless very high laser power is used). This complicates the process when it is desired to deposit pattern material that is not contiguous with pattern material that is present before the repair is initiated. Further, using known IR lasers which operate at wavelengths absorbed by fused silica is ordinarily unacceptable in as much as the long wavelength precludes producing sub micron mask features.
Conventional LICVD based solely on heating of the substrate suffer from the drawback that as time is allowed to pass while the laser is trained on the point, in order to deposit a sufficient thickness of material over the area, the size of the area which has a sufficient temperature to cause decomposition of the compound grows, which in turn leads to undesired lateral spread of the deposited metal. Semiconductor applications have very low tolerance for dimensional errors, and therefore undesired lateral growth is unacceptable for semiconductor applications.
Another type of conventional LICVD is photolytic in character, in that an organometallic molecule is photochemically, as opposed to thermally, decomposed at or near a substrate surface by a laser operating at a wavelength corresponding to a photon energy which exceeds a chemical bond energy of the molecule. One known photolytic process has a linear dependence on laser light intensity. A photolytic process may require a number of separate molecule-photon interactions in order to break a number of bonds in order to release elemental metal for deposition. In these processes, a portion of the photon energy which exceeds a particular molecular bond energy, may initially be converted to vibrational energy of the organometallic molecule, and subsequently be dissipated to surrounding gas molecules as heat. The excess energy therefore, does not contribute to breaking other bonds of the molecule. Therefore there is an inefficiency in the utilization of photon energy in this conventional photolytic LICVD.
In so far as semiconductor applications require tight tolerances to be maintained, in LICVD for semiconductor application, diffraction limited optical systems, including inter alia, expensive aspheric lenses are called for. For conventional LICVD, the optical system may be required to operate in the ultraviolet region of the spectrum, depending on the bond energies of the specific organometallic compound being used. The optical system must then be designed to operate in the ultraviolet which further increases its cost.
What is needed is a method and apparatus for conducting high resolution, selective deposition.
It would be desirable to have a method and apparatus for conducting LICVD that makes more efficient use of photon energy.